The present invention relates generally to wireless communication systems and, more particularly, to mobile station receiver architectures and methods that employ decoding multiple hypotheses such as in case of 3rd Generation Partnership Project (“3GPP”) Long Term Evolution (“LTE”) wireless communication system.
As shown in FIG. 1, a wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. As illustrated, the communication path from the base station (“BS”) to the client terminal direction is referred to herein as the downlink (“DL”) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (“UL”). In some wireless communication systems the client terminal or mobile station (“MS”) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
Most wireless communication systems have an overhead in terms of managing and controlling the wireless link between the network and the client terminal. A number of beacon signals may need to be transmitted by the base station that enables the client terminal to detect the base station and synchronize to it. For example, in LTE wireless communication system the Primary Synchronization Signal (“PSS”), the Secondary Synchronization Signal (“SSS”), and Physical Broadcast Channel (“PBCH”) are used to enable the client terminal to detect and synchronize to the base station. Even after the client terminal detects and synchronizes with the base station, it needs additional information about the detailed structure of the channel and various parameters required for communication with the base station in both DL and UL. This information is generally referred to as “System Information.” Depending on the particular wireless communication system, the base station may transmit the System Information in one or more smaller independent units of information.
Even after the client terminal has detected the base station, synchronized to it and had decoded the System Information, it does not have any specific resources allocated to it for communication. For this purpose, it has to first transmit a signal in UL to request resources in DL, UL, or both. In many wireless communication systems, multiple client terminals use the same resources for communication. The base station manages the overall allocation of the resources to the multiple client terminals contending for the same shared channel. While making resource allocation decisions, the base station considers a number of factors such as the required bit rate, latency, quality of service required, bit error rate, channel conditions, the loading of the cell in terms of number of active users, etc. Furthermore, these factors vary continuously and the base station adapts its decisions dynamically.
Conventional wireless communication systems are primarily used for voice calls and text messaging. The resource allocation for a voice can be done once and then it does not change for a relatively long time. For example, average phone call duration may be in the order of minutes. Similarly, text messaging may be much less frequent. The latency in allocation of resources for setting up a voice call is usually in the order of several seconds. This latency is generally acceptable to the users since it is a onetime latency during the call setup. After the call is setup the allocated channel resources are dedicated to the user for the duration of the call. Another part of the resource allocation is the overhead incurred in the process of allocating the resources. For example, number and size of control messages required to establish a phone call may be significant. However, the call setup overhead due to control message is a small percentage of the duration of the call since the overhead is incurred only once per call.
Over the years, the use of the internet has increased over the wireless communication networks. Normally the traffic pattern of internet usage is very bursty, e.g., the request for resources comes very frequently but each request is only for a short duration of time. Under such traffic conditions the latency of several seconds in the conventional wireless communication systems may not be acceptable. Therefore, one requirement is to have a resource allocation method that can allocate resources with low latency. Another requirement is that the allocation of the resources must incur low overhead since the allocation, release and reallocation of resources occur much more frequently.
The LTE wireless communication system is designed for low latency and high throughput applications. Examples of such applications include the web browsing, mobile online gaming, video calls, media streaming, etc. Supporting such applications requires the allocation of resources in a dynamic manner. This is in contrast with respect to the previous generation wireless communication systems that are designed for allocations that do not change for tens of seconds and even minutes and hours. In LTE wireless communication system, the resource allocation may change once every millisecond.
The potential penalty for such dynamic resource allocation may be that the overhead for allocating the resources is incurred every millisecond. To keep the overhead of resource allocation low while keeping the resource allocation dynamic and the latency low, the LTE wireless communication system employs several techniques.
The LTE wireless communication system employs Orthogonal Frequency Division Multiple Access (“OFDMA”) technology in the DL air interface. The basics of OFDMA are described in “4G LTE/LTE-Advanced for Mobile Broadband” by Dahlman, Erik, et al., copyright 2011 and published by Academic Press, MA, the entire disclosure of which is hereby expressly incorporated by reference herein. The high level structure of the LTE DL air interface, as described in 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (“E-UTRA”); Physical channels and modulation,” is shown in FIG. 2. The air interface consists of series of frames of 10 ms each and each frame consists of 10 subframes with 1 ms per subframe. As shown in FIG. 3 each subframe in turn consists of 12 or 14 OFDM symbols depending on the length of Cyclic Prefix (“CP”) used. The FIG. 3 shows the structure for Normal CP with 14 OFDM symbols per subframe for the case of 10 MHz channel bandwidth with 50 Resource Blocks (“RBs”). FIG. 3A focuses on certain subframes from FIG. 3 for clarity purposes.
The first few OFDM symbols of each subframe are used for control channel purposes and it is called Control Region as shown in FIG. 3. A control channel, called Physical Downlink Control Channel (“PDCCH”) is designed for the purpose of dynamic resource allocation. The payload data describing the resource allocation information that is transmitted using PDCCH is called Downlink Control Information (“DCI”). The DCI describes the allocation of the resources in the remaining portion of the subframe call Data region.
The PDCCH is transmitted within the control region of each subframe. The number of OFDM symbols used for the control region may vary from one subframe to another. The actual number of OFDM symbols used for a subframe is given by another control channel called Physical Control Format Indicator Chanel (“PCFICH”). The PCFICH is always transmitted in the first OFDM symbol of each subframe. The number of control symbols in each subframe is at least one OFDM symbol. Each PDCCH allocates resources for one client terminal in either DL or UL. Therefore, there may be multiple PDCCHs in the control region.
In LTE wireless communication systems the base station is referred as Enhanced Node B (“eNB”). One of the requirements from eNB in LTE wireless communication systems is the flexibility in addressing (sending resource allocation to) a particular client terminal through the PDCCH. This flexibility in turn requires the client terminal to search all the possible PDCCH candidates within different parts of the control region in each subframe, as shown in FIG. 3, for possible resource allocation to it. In any given subframe, there may or may not be any resource allocation for a particular client terminal. The allocation for DL and UL are provided separately since the internet traffic pattern in general may be asymmetric. Therefore, in a single subframe there may be zero, one or two PDCCHs transmitted by the base station to a particular client terminal. In some special conditions, there may be more than two PDCCHs transmitted to a particular client terminal in a single subframe.
To keep the allocation overhead low, the PDCCH may be transmitted with different level of Forward Error Correction (“FEC”). This is referred to as Aggregation Level (“AL”) in LTE wireless communication systems. Depending on the expected signal conditions of the client terminal to which the PDCCH is transmitted, the base station may dynamically use a different AL. However, the client terminal may not be a priori aware of the AL used by the base station. The AL used for different client terminals may be different. A Control Channel Elements (“CCE”) consists of 72 transmission bits (coded bits after FEC) and it is a basic allocation unit for PDCCH transmission within a subframe. Each aggregation level uses one or more CCEs within the control region of a subframe. There are total of four different aggregation levels used in LTE wireless communication systems as shown in FIG. 4, employing 1, 2, 4, and 8 CCEs with 72, 144, 288 and 576 transmission bits respectively.
In LTE wireless communication systems different formats for the DCI messages are used for handling different allocation requirements. For example, DL allocation and UL allocation messages may have different types of information. In LTE wireless communication systems different multi-antenna transmission modes are used. Depending on the particular transmission mode used the type and length of the DCI messages may vary. At any given time, a UE (user equipment) is required to decode DCI messages of at most two possible different lengths.
Considering all the above factors, the PDCCH AL, the length of the DCI message and all the possible PDCCHs that may be transmitted by the base station, the client terminal has to perform PDCCH decoding with a number of different combinations. This is often referred to as blind PDCCH decoding.
In LTE wireless communication systems different UEs are identified using various identities known Radio Network Temporary Identifier (“RNTI”) which is unique within a cell. There are some RNTIs that are broadcast type which address all the UEs in a cell whereas there are other RNTIs that address a particular client terminal. Each client terminal is assigned a unique RNTI within the cell when it first camps on a cell. In a PDCCH, a particular client terminal is addressed by the eNB by using the RNTI for that client terminal. However, in order to keep the overhead low, the RNTI is not explicitly added to the DCI payload.
A concept of search space is used in LTE wireless communication systems to reduce the number of PDCCH candidates that a client terminal must decode in each subframe. The search space is divided into two parts: Common Search Space (“CSS”) and UE Specific Search Space (“UESSS”). The PDCCHs with broadcast RNTIs may be transmitted only in the CSS whereas the PDCCH with UE specific RNTI may be transmitted in either the CSS or UESSS. The CSS is common to all the UEs that are camped on a cell. The UESSS is derived from the UE specific RNTI. Within the control region of each subframe, the UE only searches within the CSS and its UESSS for possible PDCCHs being transmitted to it. The specific CCEs to which a particular PDCCH is mapped to is a function of the search space, aggregation level and the RNTI of the UE in case of UESSS. For the CSS, the PDCCHs are always mapped to the first 16 CCEs as shown in FIG. 5. The mapping of the UESSS PDCCH candidates depends on its RNTI and an example of that is shown in FIG. 6. The summary of the PDCCH candidates a UE is required search under normal operation is summarized in the table contained in FIG. 7. Considering all the PDCCH candidates and two different DCI lengths, total of 44 blind PDCCH decoding attempts may be required in each subframe.
In addition to the FEC, error detection is used for PDCCH to enable the client terminal to ensure whether the PDCCH decoding is successful or not. The error detection is performed using a 16-bit Cyclic Redundancy Check (“CRC”). The RNTI of the client terminal to which the PDCCH is addressed is XOR-ed with the computed CRC over the DCI payload as shown in FIG. 8. The intended RNTI may be a broadcast RNTI or client terminal specific RNTI.
During the course of blind PDCCH decoding the client terminal must match the locally computed CRC against one of the broadcast RNTIs or its assigned unique RNTIs. Only when the XOR-ed CRC match, a PDCCH decoding is considered successful.
During blind PDCCH decoding in the client terminal, the input to the PDCCH decoder may be from an actual signal transmitted by eNB or from some random values from parts of the downlink signal. This may be because the eNB may not be transmitting any information at all or may be transmitting information intended for other client terminals. Only few (typically two or three) of the 44 blind decoding attempts may have a useful signal transmitted by eNB intended for the particular client terminal as input to the PDCCH decoder. The probability that a random 16-bit pattern matches the true CRC for the payload portion of the data is 1/216. When a computed CRC on the received PDCCH matches the received CRC when there was no PDCCH transmitted, it is defined as a false PDCCH decoding. Considering that there are 44 blind decoding attempts made by the client terminal per subframe, the probability of getting a false PDCCH decoding per subframe is 44*(1/216). Furthermore, the PDCCH CRC is checked in conjunction multiple RNTIs that may be used by the client terminal. Assuming that on average two identifiers may be used by the client terminal at any given time, the probability of false PDCCH detection may be increased further by a factor of two, i.e., 2*44*(1/216). Since there are 1000 subframes in one second, the probability of getting one false PDCCH per second is 1000*2*44*(1/216). This translates to about a 134% chance of one false PDCCH decoding per second. This means that at least one false PDCCH is likely to occur every second. When a UE is required to decode PDCCH with additional RNTIs such as SI-RNTI or SPS-RNTI, the probability of false PDCCH detection is further increased.
The false PDCCH decoding may lead to false DCI which in turn leads to false resource allocation in the client terminal. Such false PDCCH detection may cause two types of problems. If the false PDCCH detection is related to downlink resource allocation then it may cause the client terminal to receive the downlink data that does not actually contain any information for that particular client terminal. This results in unnecessary power consumption in the client terminal. Furthermore, if there was another allocation in the same subframe that was actually intended for the client terminal there may be conflict in the allocated resources. This may cause the client terminal to behave in an unpredictable manner and could result in the client terminal not receiving the data that was intended for it. For the uplink direction, the false detection of the PDCCH may result in the client terminal transmitting on resources that are not granted to it. This may cause interference to other client terminal which may be allocated those particular resources. This may lead to unnecessary power consumption on all the client terminals that may be transmitting on those particular resources. Furthermore, the bandwidth is wasted in both the downlink and the uplink of the wireless communication system.
The LTE wireless communication system uses Hybrid Automatic Repeat Request (“HARQ”). False PDCCH detection can also cause the HARQ Finite State Machine (“FSM”) running at the client terminal and at the eNB to be out of sync. For each downlink resource allocation there is a corresponding HARQ acknowledgement in the uplink. The location of uplink acknowledgement is based on the start position of the PDCCH blind decoding candidate. The false PDCCH decoding then in turn leads to transmission of HARQ acknowledgment (positive or negative) in the uplink direction at the wrong location in uplink resources and possibly interfering with other client terminals that may be sending HARQ acknowledgements. The false PDCCH decoding may lead to a series of problems that compound over a period of few subframes.
Multiple successful PDCCH detection may occur when single set of PDCCH data is processed by the receiver in the client terminal assuming different message sizes and coding rates and AL. For example, it may be possible to successfully decode a message of the same size with different AL assumption. This leads to multiple successful decoding of a single PDCCH for a given client terminal. This is referred herein as duplicate PDCCH detection.